An Introvert's Hookup Hiccups: This Gyaru Is Head Over Heels for Me! Volume 6 by Yuishi

Yoshin and Nanami stumble upon a mysterious letter asking about their dare. While they’re both curious to track down the sender, they decide it best to wait and see how the situation unfolds. After all, it’s summer break—which calls for a trip to the fireworks festival while all dressed up in yukata! Even though the two try not to be too flirty while out in public, what would their relationship be without an unexpected accident that brings them even closer together? With a break packed with plans for karaoke dates and study sessions to prepare for exams, it’s seductively sweet business as usual for the summer installment of this couple’s sugar-filled romance!